Transimpedance amplifiers (TIA) are widely used in optical communications. A TIA receives a current-mode input signal and outputs a voltage-mode signal. A prior art TIA 100 is depicted in FIG. 1. TIA 100 comprises: a bias circuit 110 comprising a current source 111 for establishing a substantially constant current Ib flowing from circuit node 101 to circuit node VSS; a common-gate amplifier 120 comprising a NMOS (short for n-type metal-oxide semiconductor field effect transistor) 121, wherein a gate terminal is coupled to a bias voltage VB, a source terminal is coupled to circuit node 101, and a drain terminal is coupled to circuit node 102; and a load circuit 130 comprising a resistor 131 coupling between circuit node VDD and circuit node 102. Throughout this disclosure, VSS denotes a first substantially fixed-potential circuit node that is commonly referred to as a “ground node”; and VDD denotes a second substantially fixed-potential circuit node that is commonly referred to as a “power supply node.” A current-mode input signal is injected into circuit node 101, while a voltage-mode output signal is generated at circuit node 102. The input current is denoted as Ii and the resistance value of resistor 131 is denoted as R.
Note that an output current of a current source is substantially constant unless an output voltage changes too fast or too much. Assuming the input current is sufficiently small and changes sufficiently slow, then an output current Io of the common-gate amplifier 120 at circuit node 102 will be approximately Ib−Ii, and therefore the output voltage will be approximately VDD−(Ib−Ii)·R, indicating the small-signal gain of TIA 100 is approximately equal to R. However, if the input current changes too rapidly, current source 111 may not be able to keep a constant output current. The gain of TIA 100, in other words, is band limited. To increase the speed of TIA 100, requires the common-gate amplifier 120 to have a higher transconductance, which requires a higher bias current, i.e. a larger Ib. With a higher bias current, however, the resistance value of resistor 131 needs to be smaller due to the limited headroom constrained by the voltage difference between VDD and VSS. This would prevent TIA 100 from having a high gain, as the gain of TIA 100 is proportional to the resistance value of resistor 131. In summary, there is a tight constraint on the gain of TIA 100 due to limited voltage headroom when one seeks to increase the speed of TIA 100, since the voltage headroom used by the load circuit 130 is proportional to the bias current Ib.
What is desired is a TIA with more headroom for higher gain when one seeks to increase the speed.